Hitherto, as the element member of semiconductor devices, photosensitive devices for use in electrophotography, image input line sensors, image pickup devices or other optical devices, there have been proposed a number of amorphous semiconductor films, for instance, an amorphous deposited film composed of an amorphous silicon material compensated with hydrogen atoms (H) or/and halogen atoms (X) such as fluorine atoms or chlorine atoms hereinafter referred to as "a-Si(H,X)"]. Some of such films have been put to practical use.
Along with those amorphous semiconductor films, there have been proposed various methods for their preparation using plasma chemical vapor deposition technique wherein a raw material is decomposed by subjecting it to the action of an energy of direct current, high frequency or microwave glow discharging to thereby form a deposited film on a substrate of glass, quartz, heat-resistant resin, stainless steel or aluminum. And there have been also proposed various apparatus for practicing such methods.
Now, in recent years, the public attention has been focused on plasma chemical vapor deposition process by means of microwave glow discharging decomposition hereinafter expressed by the abbreviation "MW-PCVD process" also at industrial level.
A typical MW-PCVD apparatus for practicing the MW-PCVD process is disclosed in Japanese Laid-open Patent Application No. 59(1984)-078528. Japanese Laid-open Patent Application No. 61(1986)-283116 discloses an improved MW-PCVD apparatus. In the latter publication, there is described a MW-PCVD process wherein an electrode is provided in the discharge space and a desired voltage is applied during the film formation in order to control the plasma potential in the discharge space, thereby depositing a film while controlling ion impacts of deposition species. It is also described how the characteristics of the resulting deposited film are substantially improved.
Typical examples of the foregoing known MW-PCVD apparatus are shown in FIG. 4 and FIG. 5.
FIG. 4 shows a schematic constitution of the known MW-PCVD apparatus of the type wherein only one substrate is provided. FIG. 5 shows a schematic constitution of the known MW-PCVD apparatus of the type wherein a plurality of substrates are provided simultaneously.
In FIG. 4, reference numeral 401 indicates a reaction chamber having a hermetically sealed vacuum structure. The reaction chamber is provided with a microwave introducing window 402 formed of a dielectric material through which microwave power can be efficiently transmitted into the reaction chamber. The dielectric material of which the microwave introducing window is formed is, for example, quartz glass, alumina ceramics or the like. Reference numeral 403 indicates a microwave transmitting unit comprising a metallic waveguide extending through a matching box and an isolator from a microwave power source (not shown). The reaction chamber 401 is provided with an exhaust pipe 404 which is open into the reaction chamber at one end and is communicated through an exhaust valve with an exhaust device (not shown) at the other end. Reference numeral 405 indicates a substrate on which a deposited film is formed which is placed on a substrate holder 407 having an electric heater (not shown) for controlling the temperature of the substrate 405. The reaction chamber 401 is provided with a gas supply pipe 408 extending from a reservoir (not shown). Reference numeral 406 indicates a discharge space. Reference numeral 410 indicates an electrode situated in the discharge space 406 which serves to control plasma potential of the discharge space. The electrode 410 is electrically connected to a bias power source 409 such as DC power source.
The apparatus shown in FIG. 5 is similar to that of FIG. 4 except that a plurality of substrates 505 are provided along a bias electrode 510.
The film formation by these known MW-PCVD apparatus is fundamentally carried out in the following manner, for example, in the case of using the apparatus of FIG. 4.
The reaction chamber 401 is evacuated by means of a vacuum pump (not shown) to bring the inner pressure to about 1.times.10.sup.-7 Torr. Subsequently, the heater built in the substrate holder 407 is energized to maintain the substrate 405 at a desired temperature suitable for the formation of a deposited film. When an amorphous silicon deposited film is formed, for example, a starting gas such as silane gas (SiH.sub.4) is fed into the reaction container 401 through the gas supply pipe 408. At the same time, the microwave power source is turned on so that a microwave having a frequency of 500 MHz or over, preferably 2.45 GHz, is generated. This microwave is passed through the waveguide 403 and the microwave introducing window 402 into the reaction chamber 401.
Thus, the gas in the reaction chamber 401 is excited with the action of microwave energy and dissociated, thereby causing the formation of a deposited film on the surface of the substrate 405.
Any of the foregoing known MW-PCVD apparatus makes it possible to form a relatively thick photoconductive film at a high deposition rate.
With such known apparatus, however, when a large area device is made, such as an electrophotographic photosensitive member which is required to have uniform characteristics over a large area, satisfactory results are difficult to obtain in view of the characteristics and economy.
More particularly in this respect, when a starting gas is fed from the outside of the discharge space, the gas is successively decomposed and ionized when placed in plasma zone. At this time, the radical concentration and the ion concentration in the plasma zone become greatly varied at the upstream side (the inlet side of the starting gas) and at the downstream side (exhaust means side of the starting gas) of the starting gas.
This entails variations in the quantity and intensity of ion bombardment on the substrate at the upstream and at the downstream sides, thereby causing a thickness and electric characteristics of the resulting deposited film to become non-uniform.
Further, since the starting gas is passed from the outside of the discharge space into the inside of the plasma, part of the gas will not pass through the central portion of the plasma where the plasma intensity is high, and will be exhausted without being decomposed. Thus, the utilizing efficiency of the starting gas unavoidably becomes insufficient.
In addition, since the starting gas is supplied from the outside of the plasma, its ionization proceeds, in most case, at a position which is far away from the bias electrode located in the inside of the plasma. The energy of the ions is not sufficiently high relative to the substrate, resulting in unsatisfactory bombardment on the substrate. When the voltage at the bias electrode increases for the purpose of enhancing the effect of the bombardment, abnormal discharge from the bias electrode, such as sparks, takes place.
On the other hand, when the starting gas supply means is provided in the inside of plasma, it invariably passes through the center of the plasma, so that decomposition proceeds satisfactorily. However, the electric field in plasma is disturbed by the starting gas supply means and by the flow of the starting gas, with the thickness of a deposited film on the substrate and the uniformity of electric characteristics being unsatisfactory.
Especially, when a plurality of substrates are provided to surround the plasma therewith in order to increase the gas utilizing efficiency, the negative influence becomes considerable.
The above influence is problematic not only when the starting gas supply means is electrically conductive, but also when it is insulative. That is, as a film becomes deposited on the surface of the starting gas supply means, the disturbance of the electric field in plasma is turned greater.